Why does javascript's "in" operator return true when testing if 0 exists in an array that doesn't contain 0?

Why does the "in" operator in Javascript return true when testing if "0" exists in array, even when the array doesn't appear to contain "0"?

For example, this returns true, and makes sense:

var x = [1,2];
1 in x; // true

This returns false, and makes sense:

var x = [1,2];
3 in x; // false

However this returns true, and I don't understand why:

var x = [1,2];
0 in x;

Solution 1:

It refers to the index or key, not the value. 0 and 1 are the valid indices for that array. There are also valid keys, including "length" and "toString". Try 2 in x. That will be false (since JavaScript arrays are 0-indexed).

See the MDN documentation.

Solution 2:

The in operator doesn't do what you're thinking it does. The in operator returns true if the specified operand is a property of the object. For arrays, it returns true if the operand is a valid index (which makes sense if think of arrays as a special-case object where the properties are simply named 0, 1, 2, ...)

For example, try this:

var x=[1,4,6];
alert(2 in x);

It'll also return true, because "2" is a valid index into the array. In the same way, "0" is an index into the array, so also returns true.

Solution 3:

Javascript's in operator does not check if a value is contained in an array. It checks if the object has a property or index. So var x = [4,5]; 4 in x; //false 1 in x; //true.

Because length is a property of x, "length" in x; //true